This invention relates to improved polypropylene compositions having good impact strength and stiffness. More particularly, the invention relates to blends comprising polypropylene, a hydrogenated block polymer impact modifier and crystalline polyethylene which exhibit an unexpected improvement in impact properties while maintaining high rigidity and stiffness properties, and to a method for enhancing the efficiency and effectiveness of block polymer impact modifiers for polypropylene.
A shortcoming of polypropylene for structural parts has been its notch sensitivity and consequent low impact strength. A variety of rubbery compounds has been employed to improve the impact resistance of polypropylene, however these improvements are usually gained at the expense of other properties. For example, the use of hydrogenated block copolymers to impact modify polypropylene is disclosed in U.S. Pat. No. 3,299,174. Although improvement in impact properties results without imparting an undesirable level of stress whitening, the stiffness of the polypropylene compositions is significantly lowered, to the point of becoming rubbery in character at high levels of modifier. This loss of rigidity precludes the use of these compositions in many structural applications where a combination of high impact strength and stiffness is required.
A method for retaining stiffness and rigidity while improving the impact properties of polypropylene would thus extend the utility of impact-modified polypropylene compositions.